The present invention relates to a process for the catalytic hydrogenolysis of p-substituted benzaldehydedialkylacetals to give the corresponding p-substituted benzyl alkyl ether derivatives.
The hydrogenolysis of benzaldehyde-dialkylacetals is known and leads to toluene, with elimination of both the alkoxy groups. This reaction takes place almost quantitatively, if the hydrogenolysis is carried out with palladium or platinum catalysts. This reaction is used, for example, when a benzyl radical which has been introduced as a protective group into a molecule is to be selectively removed again from the molecule (in this context, see, for example, P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press-New York and London, 1967, pages 449 to 454).
The catalysts of the benzyl cleavage are, however, completely unsuitable for eliminating only one methoxy radical from the dimethylacetal of benzaldehyde, in order to synthesize the corresponding benzyl methyl ether. It is known, however, from the state of the art to hydrogenolyze benzaldehyde-dialkylacetals to produce benzyl alkyl ethers by using cobalt carbonyl as the catalyst and reacting the acetals with a mixture of carbon monoxide and hydrogen. For example, in an article entitled "The reduction of acetals with cobalt carbonyl catalysts" in Can. J. Chem., Volume 54, 1976, pages 685 to 694, B. I. Fleming and H. I. Bolker describe the reaction of benzaldehyde-dimethylacetal (IIa), benzaldehyde-dibenzylacetal (IIb) and the p-substituted compounds 4-methoxybenzaldehyde-dibenzylacetal (IIc) and 4-chloro-benzaldehyde-dibenzylacetal (IId)
______________________________________ ##STR2## ##STR3## Reactant R.sup.1 R.sup.2 Product ______________________________________ IIa H CH.sub.3 IIIa b H ##STR4## b c .sup.-- .sub.--OCH.sub.3 ##STR5## c d Cl ##STR6## d ______________________________________
to give the benzyl methyl ether (IIIa) in 96% yield, the dibenzyl ether (IIIb) in 95% yield, the (4-methoxybenzyl) benzyl ether (IIIc) in 59% yield and the (4-chloro-benzyl) benzyl ether (IIId) in 16% yield. The catalyst used is Co.sub.2 (CO).sub.8 in benzene, and the ratio of H.sub.2 to CO is 2 to 1.
When this known process is transferred to those dimethylacetals of benzaldehydes which have an alkoxy, aryloxy or aralkoxy group in the p-position, for example, to 4-methoxy-benzaldehyde-dimethyl acetal or to 4-phenoxy-benzaldehyde-dimethylacetal, this catalytic hydrogenolysis proceeds only with low selectivity. Thus, in the hydrogenolysis of 4-methoxy-benzaldehyde-dimethylacetal (IV) with cobalt carbonyl as the catalyst, only about 10 to 12% of 4-methoxybenzyl methyl ether (Va) is obtained. As further products, 4-methoxytoluene (Vb) and (4-methoxy-phenyl)-acetaldehyde-dimethylacetal (Vc) are formed in yields of about 49 and 40% respectively: ##STR7##
It may be assumed that compound (Vb) is formed from (IV) by "benzyl ether" cleavage and compound (Vc) is formed by insertion of carbon monoxide into the acetal group, the chain being lengthened by one CH.sub.2 group by subsequent hydrogenation.
The bis-dimethylacetal of 4,4'-diphenyl etherdialdehyde (VI) which is described for the first time in German patent application No. 30 48 992.0, corresponding to U.S. application Ser. No. 333,013 filed concurrently herewith and having the title "4,4'-Diphenyl Ether-Dialdehydebis-Dimethylacetal and a Process For Its Preparation", shows a very similar behavior. In the hydrogenolysis carried out analogously to B. I. Fleming et al. (cited above), 26% of 4,4'-bis-(methoxy-methyl)-diphenyl ether (VII) are obtained, while the benzyl ether cleavage proceeds to the extent of 16% and the insertion of carbon monoxide proceeds to the extent of 20%. In addition, several further unknown products are formed, so that the course of the hydrogenolysis of the compound (VI) under known process conditions is markedly unselective. ##STR8##
Modifications of cobalt carbonyl catalysts which lead to very diverse results have also been previously described in the literature:
For example, it is stated in Organic Syntheses via Metal Carbonyls, edited by I. Wender and P. Pino, Interscience Publishers-New York, 1968, pages 84 to 86, that bases acting on cobalt carbonyl initiate the disproportionation of the latter to give ionic complexes, and this is also termed the "base reaction". In this way, the cobalt carbonyl, as a catalytically active compound of a homogeneous reaction system, can be transformed into inactive ion pairs.
In the articles "Novel Hydroformylation Catalysts" by L. H. Slaugh and R. D. Mullineaux in J. Organometal. Chem., Vol. 13, 1968, pp. 469-477, and "Homogeneous Hydrogenation of Ketones Catalyzed by Cobalt Carbonyl Phosphine Complexes" by L. Marko, B. Beil and S. Vastag in Homogeneous Catalysis-II, Advances in Chemistry Series 132 of the American Chemical Society, 1974 pp. 27-32, it is suggested, in order to control the selectivity of cobalt carbonyl catalysts, to modify the latter with certain organic phosphorus compounds. This known method, however, fails when used in the hydrogenolysis of the acetals indicated at the outset. It was not possible to achieve a hydrogenolysis of these acetals with cobalt carbonyls which contained triphenylphosphane, tri-n-butylphosphane or triethyl phosphite.